Organic EL devices, especially low-molecular organic EL (hereinafter abbreviated as “OLED”) devices were significantly improved in characteristics, such as a substantial reduction in drive voltage, by Eastman Kodak Company owing to the separation of functions as a result of the formation of an organic layer into an ultrathin film and a multilayer construction (Non-patent Document 1: Applied Physics Letters, Vol. 51, pp. 913-915, U.S.A. (1987)).
Further, organic EL (hereinafter abbreviated as “PLED”) devices making use of a high-molecular light-emitting material were developed by University of Cambridge (Non-patent Document 2: Nature, Vol. 347, pp. 539-541, Great Britain (1990)). Characteristics of high-molecular organic EL devices in recent years have been improved to such a level as favorably comparable with those of conventional OLED devices.
Concerning the above-described OLED devices, it has been reported that the arrangement of a copper phthalocyanine (CuPC) layer as a hole injection layer can improve initial-stage characteristics, such as a reduction in drive voltage and an improvement in luminescence efficiency, and further can materialize improvements in life characteristics (Non-patent Document 3: Applied Physics Letters, Vol. 69, pp. 2160-2162, U.S.A. (1996)).
With respect to PLED devices, on the other hand, it has been reported that the use of a polyaniline material (Non-patent Document 4: Nature, Vol. 357, pp. 477-479, Great Britain (1992); Applied Physics Letters, Vol. 64, pp. 1245-1247, U.S.A. (1994)) or a polythiophene material (Non-patent Document 5: Applied Physics Letters, Vol. 72, pp. 2660-2662, U.S.A. (1998)) as a hole transport layer (buffer layer) can bring about similar advantageous effects as the OLED devices.
It was also found that the use of a metal oxide (Non-patent Document 6: IEEE Transactions on Electron Devices, Vol. 44, pp. 1245-1248, U.S.A. (1997)), a metal halide (Non-patent Document 7: Applied Physics Letters, Vol. 70, pp. 152-154, U.S.A. (1997)), a metal complex (Non-patent Document 8: Japanese Journal of Applied Physics, Vol. 38, pp. L1348-1350 (1999)) or the like as an electron injection layer leads to improved initial characteristics. These charge injection layers and buffer layers have then found wide-spread utility.
Recently, a charge-transporting varnish of an organic solvent system, which makes use of a low-molecular oligoaniline material, has been developed, and the insertion of a hole injection layer obtained by using this varnish has been found to show excellent EL device characteristics (Patent Document 1: JP-A 2002-151272).
CuPC, a common hole injection material for OLED devices, however involves a drawback that it tends to result in a film having a very rough surface and its mixing in a trace amount in another organic layer leads to substantially deteriorated characteristics.
Polyaniline materials and polythiophene materials, which are currently employed in PLED devices, involve problems in that they contain as a solvent water having a potential problem of promoting a device deterioration, a limitation is imposed on usable solvents, and due to the aggregation and low solubility of the material, a limitation is also imposed on the coating method capable of forming a uniform film.
The use of a charge-transporting varnish of an organic solvent system, which contains a low-molecular oligoaniline material having high solubility, may also develop problems such that a limitation is imposed on the kind of usable electron-accepting dopants and the electron-accepting dopants are low in heat resistance and amorphousness. A charge-transporting varnish containing a charge-transporting material and charge-accepting dopant material of low molecular weights, especially a varnish containing a crystalline material may generally have a difficulty in forming a film which shows high levelness.
Patent Document 1:                JP-A 2002-151272        
Non-patent Document 1:                Applied Physics Letters, Vol. 51, pp. 913-915, U.S.A. (1987)        
Non-patent Document 2:                Nature, Vol. 347, pp. 539-541, Great Britain (1990)        
Non-patent Document 3:                Applied Physics Letters, Vol. 69, pp. 2160-2162, U.S.A. (1996)        
Non-patent Document 4:                Nature, Vol. 357, pp. 477-479, Great Britain (1992)        
Non-patent Document 5:                Applied Physics Letters, Vol. 64, pp. 1245-1247, U.S.A. (1994)        
Non-patent Document 6:                Applied Physics Letters, Vol. 72, pp. 2660-2662, U.S.A. (1998)        
Non-patent Document 7:                IEEE Transactions on Electron Devices, Vol. 44, pp. 1245-1248, U.S.A. (1997)        
Non-patent Document 8:                Japanese Journal of Applied Physics, Vol. 38, pp. L1348-1350 (1999)        